This invention relates to purification of silicon and more particularly it relates to the purification of silicon using fractional crystallization.
Because of the increasing interest in converting sunlight into electricity by use of a photovoltaic cell, there has been a considerable increase in demand for semiconductor materials such as silicon. For use in such applications, the silicon has to be quite pure, e.g. 99.99 wt. % or greater, adding greatly to its cost. Thus, for purposes of making such cells competitive with other sources of electricity, there is a great need for a method for providing high purity silicon in a highly economical manner.
In the prior art, there are disclosed several different methods for producing purified silicon. For example, Litz et al in apaper entitled "Crystallization of Pure Silicon from Molten Aluminum" presented at the Second Joint AICheE Meeting, May 19-22, 1968 (preprint 37B) disclose purifying silicon by continuous recrystallization of commercial silicon from molten aluminum using a thermal connection loop. However, Litz et al note that the drawing force for circulation of fluids in the system is the density generated by keeping one vertical portion at a different temperature from another, and that the maximum temperature difference is no more than about 20.degree. C. In U.S. Pat. No. 2,471,899, there is disclosed a method of separating constituents of alloys by fractional crystallization according to which it was recognized that a relatively high drop amounting to from 5.degree. to 20.degree. C. divides the entire volume of the melt under treatment sharply into two regions. One of them is the molten alloy, the other is the zone of the crystallized solid phase. According to the patent, the separation process can be considerably accelerated, and the purity of the liquid phase increased if during the treatment the molten alloy is kept in intensive continuous movement, and so much heat per unit of time is abstracted from the melt by means of a cooled or crystallization surface that the entire higher melting phase separates only on the crystallization surface in a relatively short time and in the form of a very compact layer. Also, in the prior art, it is disclosed (U.S. Pat. No. 3,008,887) that elemental silicon containing trace impurities can be purified by subjecting the silicon to melting within a closed reactor under anhydrous conditions and in the presence of dissociated atomic hydrogen maintained in direct contact with molten silicon being formed and by removing the vaporized reaction product impurities. According to the patent, such process is particularly suitable for purifying boron-contaminated silicon. In addition to the above, U.S. Pat. No. 2,866,701 discloses a method of purifying silicon and ferrosilicon using gaseous carbon dioxide to remove aluminum and calcium by oxidation. However, in spite of the prior art, there still remains a great need for a method for producing high purity silicon in an economical manner. The present invention fulfills that need by providing a highly economical process for purifying silicon in substantial quantity. The process can be used in upgrading silicon to a purity level in the order of about 99.99 wt.% or higher.